458 A MANUAL OF VETERINARY PHYSIOLOGY 



usually accompanying acts of stepping. It is evident that 

 impulses which stimulate contraction are passing from muscles 

 and joints to spinal centres. The spinal centres which execute 

 reflex walking, running, etc., receive much help and direction 

 from afferent arcs which arise in the labyrinth of the ear. The 

 stimuli, which are the source of reflex walking, etc., arise, there- 

 fore, in many receptive organs. 



It is by means of the deep stimuli that the proprioceptors of 

 the limbs maintain the extensor muscles in a state of tonic 

 activity, and so enable the upright position of the body to be 

 maintained without effort and without the knowledge of the 

 individual. When, therefore, the spinal dog is held in the air 

 legs downwards, gravity acting on the joints and muscles — viz., 

 weight and pressure — is the cause of impulses passing from the 

 deep fields to the spinal cord, and so maintaining the act of stepping. 

 When the animal is inverted, the deep fields no longer receive 

 their natural stimulus, and the movement ceases. We shall see 

 shortly that the chief ganglion controlling the proprioceptive 

 system is the cerebellum. 



The Standing Reflex. — When the brain of a dog or cat is removed 

 between the anterior and posterior colliculi, the extensor muscles 

 of the limbs, the extensor muscles of the neck, back, and tail, 

 and those which close the jaw, are in a condition of mild tonic 

 contraction. Their antagonists, the flexors, exhibit relaxation. 

 The phenomenon is known as Decerebrate Rigidity. The rigidity 

 of the extensor muscles is due to impulses passing out from the 

 cord as a reflex effect, for if the afferent nerves of the muscle 

 be divided the rigidity ceases. The reflex effect originates in 

 the muscle, for if the nerves of the skin be divided the rigidity 

 continues. The decerebrate preparation if stood on its legs, 

 remains there, the rigidity of the extensor muscles of the above- 

 named parts sufficing to maintain the erect attitude, even for 

 hours at a time. From the above experiment of Sherrington's 

 it is evident that the standing posture is a reflex act, and he has 

 further shown that the destruction of the labyrinth, either before 

 or after decerebration, does not prevent rigidity of the extensor 

 muscles occurring. When, however, the brain is divided behind 

 the pons the preparation can no longer remain erect, nor can a 

 decapitated dog or cat stand. The act of standing in the normal 

 animal receives full consideration in a subsequent chapter 

 on the Locomotor Apparatus. 



Other Reflex Acts. — We have seen that reflex acts are not con- 

 fined to those affecting skeletal muscles ; the act may be a 

 secretory or nutritive one, or involving the contraction or re- 

 laxation of pale muscle ; for example, the contraction and dilata- 

 tion of the bloodvessels under the influence of the vasomotor 



